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Figure 1. Breeding and year-round ranges of the Seaside Sparrow, by subspecies.

In winter, some individuals remain as far north as New England, and some move south of the mapped range in Florida and Texas. See Distribution for details; for a discussion of the subspecies shown here, see Systematics.

Seaside Sparrow.

Underparts whitish with diffuse gray streaks and strong buffy wash across breast and on sides and flanks. Crown with indistinct, diffuse central grayish stripe bounded laterally by broad brown stripes and streaked narrowly with black. Lores dusky, yellow supraloral spot blends to olive-green over eye. Ear patch gray to blackish outlined below by narrow white or pale buff malar stripe. Broad moustachial stripe gray framed by pale malar and white throat and chin; short postauricular stripe blackish, variably developed. Wing feathers would need to be examined more carefully to determine if this bird is in Formative or Definitive Basic Plumage. In fresh fall plumage, underparts show more orange-buff and upperparts tend to be browner than they are when more worn in spring, due to both wearing of basic/formative feathers and replacement with alternate feathers. The longer bill, gray supercilium, and diffuse breast streaks with central spot helps separate this from a Saltmarsh Sparrow. This individual is likely A. m. maritima, the breeding subspecies of New England.

The Seaside Sparrow is a habitat specialist of salt and brackish marshes. First described by Wilson (
Wilson, A. (1811). American Ornithology. Philadelphia, PA, Vol. no. 4:20-26.
1), it has attracted the interest of systematists since the late 1800s. Occurring in relatively small, localized populations along the Atlantic and Gulf coasts of the United States, this species has been divided into several morphologically distinct subspecies.

The Seaside Sparrow is socially monogamous, though extrapair matings have been reported from South Carolina. The species is territorial, but often feeds long distances from the defended space around its nest, a response to the wide separation of nesting and feeding areas in tidal zones it inhabits. Under ideal conditions, it may occur at high population densities, a reflection of the high productivity of salt marshes. Optimal habitat is found in marshes with expanses of medium-high cordgrass with a turf of clumped, residual stems. Especially suitable are spots not subject to extreme flooding that have open muddy areas for feeding. Nest mortality of northern populations is caused mainly by storm flooding; flooding is a significant mortality factor among southern groups, but predation is also important, and its intensity is often related to changes in water levels.